| From: | Greg Smith <greg(at)2ndquadrant(dot)com> |
|---|---|
| To: | Mark Mielke <mark(at)mark(dot)mielke(dot)cc> |
| Cc: | Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: wal_synch_method = open_sync safe on RHEL 5.5? |
| Date: | 2010-06-18 06:02:57 |
| Message-ID: | 4C1B0C11.1010204@2ndquadrant.com |
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| Lists: | pgsql-performance |
Mark Mielke wrote:
> The conclusion I read was that Linux O_SYNC behaves like O_DSYNC on
> other systems. For WAL, this seems satisfactory?
It would be if it didn't have any bugs or limitiations, but it does.
The one pointed out in the message I linked to suggests that a mix of
buffered and O_SYNC direct I/O can cause a write error, with the exact
behavior you get depending on the kernel version. That's a path better
not explored as I see it.
The kernels that have made some effort to implement this correctly
actually expose O_DSYNC, on newer Linux systems. My current opinion is
that if you only have Linux O_SYNC, don't use it. The ones with O_DSYNC
haven't been around for long enough to be proven or disproven as
effective yet.
--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us
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